Spark LDA 主题抽取

本文主要对使用Spark MLlib LDA进行主题抽取时遇到的工程问题做一总结，列出其中的一些小坑，或可供读者借鉴。关于LDA的具体理论等可以自行google。主题预测请参考：Spark LDA 主题预测

开发环境：spark-1.5.2，hadoop-2.6.0，spark-1.5.2要求jdk7+。语料有大概70万篇博客，十亿+词汇量，词典大概有五万左右的词。

训练语料代码

：apache/spark/examples/mllib/

// scalastyle:off println
package org.apache.spark.examples.mllib
import java.text.BreakIterator
import scala.collection.mutable
import scopt.OptionParser
import org.apache.log4j.{Level, Logger}
import org.apache.spark.{SparkContext, SparkConf}
import org.apache.spark.mllib.clustering.{EMLDAOptimizer, OnlineLDAOptimizer, DistributedLDAModel, LDA}
import org.apache.spark.mllib.linalg.{Vector, Vectors}
import org.apache.spark.rdd.RDD
/**
* An example Latent Dirichlet Allocation (LDA) app. Run with
* {{{
* ./bin/run-example mllib.LDAExample [options] <input>
* }}}
* If you use it as a template to create your own app, please use `spark-submit` to submit your app.
*/
object LDAExample {
private case class Params(
input: Seq[String] = Seq.empty,
k: Int = 20,
maxIterations: Int = 10,
docConcentration: Double = -1,
topicConcentration: Double = -1,
vocabSize: Int = 10000,
stopwordFile: String = "",
algorithm: String = "em",
checkpointDir: Option[String] = None,
checkpointInterval: Int = 10) extends AbstractParams[Params]
def main(args: Array[String]) {
val defaultParams = Params()
val parser = new OptionParser[Params]("LDAExample") {
head("LDAExample: an example LDA app for plain text data.")
opt[Int]("k")
.text(s"number of topics. default: ${defaultParams.k}")
.action((x, c) => c.copy(k = x))
opt[Int]("maxIterations")
.text(s"number of iterations of learning. default: ${defaultParams.maxIterations}")
.action((x, c) => c.copy(maxIterations = x))
opt[Double]("docConcentration")
.text(s"amount of topic smoothing to use (> 1.0) (-1=auto)." +
s"  default: ${defaultParams.docConcentration}")
.action((x, c) => c.copy(docConcentration = x))
opt[Double]("topicConcentration")
.text(s"amount of term (word) smoothing to use (> 1.0) (-1=auto)." +
s"  default: ${defaultParams.topicConcentration}")
.action((x, c) => c.copy(topicConcentration = x))
opt[Int]("vocabSize")
.text(s"number of distinct word types to use, chosen by frequency. (-1=all)" +
s"  default: ${defaultParams.vocabSize}")
.action((x, c) => c.copy(vocabSize = x))
opt[String]("stopwordFile")
.text(s"filepath for a list of stopwords. Note: This must fit on a single machine." +
s"  default: ${defaultParams.stopwordFile}")
.action((x, c) => c.copy(stopwordFile = x))
opt[String]("algorithm")
.text(s"inference algorithm to use. em and online are supported." +
s" default: ${defaultParams.algorithm}")
.action((x, c) => c.copy(algorithm = x))
opt[String]("checkpointDir")
.text(s"Directory for checkpointing intermediate results." +
s"  Checkpointing helps with recovery and eliminates temporary shuffle files on disk." +
s"  default: ${defaultParams.checkpointDir}")
.action((x, c) => c.copy(checkpointDir = Some(x)))
opt[Int]("checkpointInterval")
.text(s"Iterations between each checkpoint.  Only used if checkpointDir is set." +
s" default: ${defaultParams.checkpointInterval}")
.action((x, c) => c.copy(checkpointInterval = x))
arg[String]("<input>...")
.text("input paths (directories) to plain text corpora." +
"  Each text file line should hold 1 document.")
.unbounded()
.required()
.action((x, c) => c.copy(input = c.input :+ x))
}
parser.parse(args, defaultParams).map { params =>
run(params)
}.getOrElse {
parser.showUsageAsError
sys.exit(1)
}
}
private def run(params: Params) {
val conf = new SparkConf().setAppName(s"LDAExample with $params")
val sc = new SparkContext(conf)
Logger.getRootLogger.setLevel(Level.WARN)
// Load documents, and prepare them for LDA.
val preprocessStart = System.nanoTime()
val (corpus, vocabArray, actualNumTokens) =
preprocess(sc, params.input, params.vocabSize, params.stopwordFile)
corpus.cache()
val actualCorpusSize = corpus.count()
val actualVocabSize = vocabArray.size
val preprocessElapsed = (System.nanoTime() - preprocessStart) / 1e9
println()
println(s"Corpus summary:")
println(s"\t Training set size: $actualCorpusSize documents")
println(s"\t Vocabulary size: $actualVocabSize terms")
println(s"\t Training set size: $actualNumTokens tokens")
println(s"\t Preprocessing time: $preprocessElapsed sec")
println()
// Run LDA.
val lda = new LDA()
val optimizer = params.algorithm.toLowerCase match {
case "em" => new EMLDAOptimizer
// add (1.0 / actualCorpusSize) to MiniBatchFraction be more robust on tiny datasets.
case "online" => new OnlineLDAOptimizer().setMiniBatchFraction(0.05 + 1.0 / actualCorpusSize)
case _ => throw new IllegalArgumentException(
s"Only em, online are supported but got ${params.algorithm}.")
}
lda.setOptimizer(optimizer)
.setK(params.k)
.setMaxIterations(params.maxIterations)
.setDocConcentration(params.docConcentration)
.setTopicConcentration(params.topicConcentration)
.setCheckpointInterval(params.checkpointInterval)
if (params.checkpointDir.nonEmpty) {
sc.setCheckpointDir(params.checkpointDir.get)
}
val startTime = System.nanoTime()
val ldaModel = lda.run(corpus)
val elapsed = (System.nanoTime() - startTime) / 1e9
println(s"Finished training LDA model.  Summary:")
println(s"\t Training time: $elapsed sec")
if (ldaModel.isInstanceOf[DistributedLDAModel]) {
val distLDAModel = ldaModel.asInstanceOf[DistributedLDAModel]
val avgLogLikelihood = distLDAModel.logLikelihood / actualCorpusSize.toDouble
println(s"\t Training data average log likelihood: $avgLogLikelihood")
println()
}
// Print the topics, showing the top-weighted terms for each topic.
val topicIndices = ldaModel.describeTopics(maxTermsPerTopic = 10)
val topics = topicIndices.map { case (terms, termWeights) =>
terms.zip(termWeights).map { case (term, weight) => (vocabArray(term.toInt), weight) }
}
println(s"${params.k} topics:")
topics.zipWithIndex.foreach { case (topic, i) =>
println(s"TOPIC $i")
topic.foreach { case (term, weight) =>
println(s"$term\t$weight")
}
println()
}
sc.stop()
}
/**
* Load documents, tokenize them, create vocabulary, and prepare documents as term count vectors.
* @return (corpus, vocabulary as array, total token count in corpus)
*/
private def preprocess(
sc: SparkContext,
paths: Seq[String],
vocabSize: Int,
stopwordFile: String): (RDD[(Long, Vector)], Array[String], Long) = {
// Get dataset of document texts
// One document per line in each text file. If the input consists of many small files,
// this can result in a large number of small partitions, which can degrade performance.
// In this case, consider using coalesce() to create fewer, larger partitions.
val textRDD: RDD[String] = sc.textFile(paths.mkString(","))
// Split text into words
val tokenizer = new SimpleTokenizer(sc, stopwordFile)
val tokenized: RDD[(Long, IndexedSeq[String])] = textRDD.zipWithIndex().map { case (text, id) =>
id -> tokenizer.getWords(text)
}
tokenized.cache()
// Counts words: RDD[(word, wordCount)]
val wordCounts: RDD[(String, Long)] = tokenized
.flatMap { case (_, tokens) => tokens.map(_ -> 1L) }
.reduceByKey(_ + _)
wordCounts.cache()
val fullVocabSize = wordCounts.count()
// Select vocab
//  (vocab: Map[word -> id], total tokens after selecting vocab)
val (vocab: Map[String, Int], selectedTokenCount: Long) = {
val tmpSortedWC: Array[(String, Long)] = if (vocabSize == -1 || fullVocabSize <= vocabSize) {
// Use all terms
wordCounts.collect().sortBy(-_._2)
} else {
// Sort terms to select vocab
wordCounts.sortBy(_._2, ascending = false).take(vocabSize)
}
(tmpSortedWC.map(_._1).zipWithIndex.toMap, tmpSortedWC.map(_._2).sum)
}
val documents = tokenized.map { case (id, tokens) =>
// Filter tokens by vocabulary, and create word count vector representation of document.
val wc = new mutable.HashMap[Int, Int]()
tokens.foreach { term =>
if (vocab.contains(term)) {
val termIndex = vocab(term)
wc(termIndex) = wc.getOrElse(termIndex, 0) + 1
}
}
val indices = wc.keys.toArray.sorted
val values = indices.map(i => wc(i).toDouble)
val sb = Vectors.sparse(vocab.size, indices, values)
(id, sb)
}
val vocabArray = new Array[String](vocab.size)
vocab.foreach { case (term, i) => vocabArray(i) = term }
(documents, vocabArray, selectedTokenCount)
}
}
/**
* Simple Tokenizer.
*
* TODO: Formalize the interface, and make this a public class in mllib.feature
*/
private class SimpleTokenizer(sc: SparkContext, stopwordFile: String) extends Serializable {
private val stopwords: Set[String] = if (stopwordFile.isEmpty) {
Set.empty[String]
} else {
val stopwordText = sc.textFile(stopwordFile).collect()
stopwordText.flatMap(_.stripMargin.split("\\s+")).toSet
}
// Matches sequences of Unicode letters
private val allWordRegex = "^(\\p{L}*)$".r
// Ignore words shorter than this length.
private val minWordLength = 3
def getWords(text: String): IndexedSeq[String] = {
val words = new mutable.ArrayBuffer[String]()
// Use Java BreakIterator to tokenize text into words.
val wb = BreakIterator.getWordInstance
wb.setText(text)
// current,end index start,end of each word
var current = wb.first()
var end = wb.next()
while (end != BreakIterator.DONE) {
// Convert to lowercase
val word: String = text.substring(current, end).toLowerCase
// Remove short words and strings that aren't only letters
word match {
case allWordRegex(w) if w.length >= minWordLength && !stopwords.contains(w) =>
words += w
case _ =>
}
current = end
try {
end = wb.next()
} catch {
case e: Exception =>
// Ignore remaining text in line.
// This is a known bug in BreakIterator (for some Java versions),
// which fails when it sees certain characters.
end = BreakIterator.DONE
}
}
words
}
}
// scalastyle:on printl

执行命令：

“` bash

spark-submit

–class “LDAExample”

–master local[*]

–driver-memory 32g

target/pack/lib/project.jar

“file:/tmp/documents”

–stopwordFile “file:/tmp/stopwords”

–k 50

–algorithm online

–maxIterations 50

–vocabSize 50000

遇到的坑

sbt pack
代码使用sbt 编译，然后提交到spark执行，所以需要打包程序所有依赖

–driver-memory
由于在master处指定了local[*] ，所以此处需要根据训练样本大小设置该参数，否则会内存溢出，如果是yarn或者mesos，则改为设置executor-memory。

–stopwordFile
可以先训练出词典，然后剔除其中不要的词，放入stopwordFile即可，词典对于最终的topic影响很大，所以尽量剔除干扰词。

–k
topic数量，越大则对内存要求越大，执行时长也相应增大

–algorithm
当前支持em和online两种，前者训练出来的是DistributedLDAModel，包含丰富的样本信息，但目前不能直接预测新文档(可以调用toLocal转换为LocalLDAModel)。后者是LocalLDAModel，可以用来预测新文档。online是后来加入的算法，性能更好。gibbs sampling 可能后续推出

–maxIterations
越大则内存和时长越大

–vocabSize
词典最大包含词数

maxResultSize
在程序中设定，存储处理结果，样本数量比较大的话，默认内存是不够的。

SparkConf().set(“spark.driver.maxResultSize”, “5g”)

–docConcentration and topicConcentration
前者为文档对主题的先验概率，后者为主体对词的先验概率，默认为-1，则系统自动赋值。见参考4

docConcentration赋值

* Optimizer-specific parameter settings:

* - EM

* - Value should be > 1.0

* - default = (50 / k) + 1, where 50/k is common in LDA libraries and +1 follows

* Asuncion et al. (2009), who recommend a +1 adjustment for EM.

* - Online

* - Value should be >= 0

* - default = (1.0 / k), following the implementation from

* [[]]https://github.com/Blei-Lab/onlineldavb]].

topicConcentration赋值

* Optimizer-specific parameter settings:

* - EM

* - Value should be > 1.0

* - default = 0.1 + 1, where 0.1 gives a small amount of smoothing and +1 follows

* Asuncion et al. (2009), who recommend a +1 adjustment for EM.

* - Online

* - Value should be >= 0

* - default = (1.0 / k), following the implementation from

* [[]]https://github.com/Blei-Lab/onlineldavb]].

文档预处理
注意训练集每行是一个源文档。SimpleTokenizer 将每行切分为词组，在此处可以通过stopwordFile来过滤词组。在训练集预处理函数preprocess中，wordCounts包含训练集中所有的词及其词频，可理解为map，并且被倒序排序，然后取vocabSize个词作为词典。将词典输出，高频词在前，可以将其中的干扰词或者不重要的词放入stopwordFile，这样反复训练几次，词典的质量就会比较高。参考1和2中训练了维基百科中500万篇文档，最后取词也就一万左右，词典质量越高，topic质量也就越高。

模型使用

训练结束,可以在模型上调用save方法保存模型,已备后续使用.

通过训练模型,可以查看不同topic在词典上的分布,以及训练样本的主题分布.

LocalLDAModel包含了topicsMatrix, 是一个vocabSize x k 矩阵.实际上给出了k个主题在词典上的分布.此处矩阵只存储了单词的索引,所以后续使用的话,需要自己保存词典,并且确保索引与该矩阵一致.在预处理训练样本的时候,每篇文档都被处理成”词索引<->词频”向量.

describeTopics(maxTermsPerTopic: Int)可以指定每个topic返回的词数量(已经按照权重降序排列),返回所有主题.

具体如何使用，用户可以参考spark 中LocalLDAModel和DistributedLDAModel的api文档。

参考:

1.https://databricks.com/blog/2015/03/25/topic-modeling-with-lda-mllib-meets-graphx.html

2.https://databricks.com/blog/2015/09/22/large-scale-topic-modeling-improvements-to-lda-on-spark.html

3.https://github.com/apache/spark/blob/master/examples/src/main/scala/org/apache/spark/examples/mllib/LDAExample.scala

4./article/1365677.html

5.http://spark.apache.org/docs/latest/quick-start.html